Information transfer over an optical fiber transmission system can be increased by optical wavelength division multiplexing (WDM). In WDM systems, a number of different wavelength optical signals, known as "channels," are transmitted in parallel along a single fiber. Multiplexing requires that there be a way of conveniently producing optical energy at different wavelengths corresponding to the channels of a WDM system. To produce a cost effective WDM system, components must be fabricated to provide reproducible and exact channel spacing.
An ideal source of such optical energy is a laser. A laser which is potentially suitable for WDM applications is the tunable semiconductor laser. Such a laser is tuned by injecting current into part of the laser cavity, which injection changes the refractive index of the waveguide. However, the tuning range is limited and exact tuning versus current characteristics are difficult to predict and are subject to aging. Woodward et al., "Effects of Aging on the Bragg Section of DBR Laser," IEEE Photon. Tech. Lett., 5(7) at 750-52 (1993).
Another approach for producing a suitable WDM energy source is to monolithically integrate optical amplifiers with a planar optical multiplexer on a semiconductive wafer. The optical cavity, which includes a multiplexer and a gain section, is defined by two cleaved facets. When one of the amplifiers is turned on, it will receive optical feedback that has been filtered by the multiplexer. Above the lasing threshold, lasing will occur at the cavity resonant wavelength nearest peak filter transmission. This wavelength will shift by exactly one multiplexer channel spacing if the neighboring amplifier is pumped instead. The device can be digitally tuned to the desired wavelength channel by driving the appropriate gain section. Tuning is now no longer limited by the obtainable change in refractive index, but only by the gain bandwidth of the amplifier. Furthermore, channel spacings are very accurate because each individual amplifier "sees" the same diffractive element.
The multiple array grating integrated cavity (MAGIC) laser is an example of the above described laser. See Soole et al., "Multistripe Array Grating Integrated Cavity (MAGIC) Laser: A New Semiconductor Laser for WDM Applications," Elect. Lett., 28(19) at 1805-07 (1992); Soole et al., "Wavelength-selectable Laser Emission from a Multistripe Array Grating Integrated Cavity Laser," Appl. Phys. Lett., 61(23) at 2750-52 (1992); Poguntke et al., "Simultaneous Multiple Wavelength Operation of a Multistripe Array Grating Integrated Cavity Laser," Appl. Phys. Lett., 62(17) at 2024-26 (1993). As described in these papers, this device uses a curved mirror grating as the wavelength selective element. This laser is said to achieve exact channel spacings and tuning ranges. However, the device only operates under pulsed current injection and has a relatively small background spontaneous emission suppression of 20-25 dB. These limitations may be due to the high loss of the curved mirror grating. Losses may occur because of the difficulty in producing a smoothly curved mirror. Losses tend to increase with increasing roughness of the mirror.
Thus the performance of these lasers for WDM applications has been limited in terms of threshold current, tuning speed, frequency selectivity or tuning range.
It is known to use a Dragone router or waveguide as a wavelength selective intracavity filter in place of the curved mirror grating. See related application Ser. No. 08/019,952; Dragone, "An N.times.N Optical Multiplexer Using a Planar Arrangement of Two Star Couplers," IEEE Photon. Tech. Lett., 3(9) at 812-15 (1991) and Glance, et al., "Applications of the Integrated Wavelength Router," submitted for publication in the Journal of Lightwave Technology. The Dragone router utilized in Ser. No. 08/019,952 may be described as a transmissive Dragone router. The transmissive Dragone router includes two free space regions connected by an optical grating comprised of unequal length waveguides. Each free space region is also connected to another plurality of waveguides, which waveguides are not part of the transmissive Dragone router. Each of these other plurality of waveguides contain optical amplifiers. The optical amplifiers connect these other waveguides, located at each end of the transmissive Dragone router, to a cleaved facet formed in the semiconductive wafer on which the aforementioned elements are formed. The two cleaved facets comprise reflective mirrors defining a cavity in which lasing action can be supported. Different laser wavelengths can be selected by exciting one amplifier from each of the plurality of waveguides.